My /boot partition is nearly full and I get a warning every time I reboot my system. I already deleted old kernel packages (linux-headers...), actually I did that to install a newer kernel version that came with the automatic updates. After installing that new version, the partition is nearly full again. So what else can I delete? Are there some other files associated to the old kernel images?

It's worth mentioning that not every Ubuntu installation will have a separate /boot partition - often it will all just be one big partition. This answer applies for those who do have a separate /boot partition. Those using LVM or "full disk" encryption need a separate /boot, otherwise it may be optional.
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thomasrutterOct 1 '14 at 3:38

Removing the linux-image-x.x.x-x package will also remove linux-image-x.x.x-x-generic.

The headers are installed into /usr/src and are used when building out-tree kernel modules (like the proprietary nvidia driver and virtualbox). Most users should remove these header packages if the matching kernel package (linux-image-*) is not installed.

To list all installed kernels, run:

dpkg -l linux-image-\* | grep ^ii

One command to show all kernels and headers that can be removed, excluding the current running kernel:

It selects all packages named starting with linux-headers-<some number> or linux-image-<some number>, prints the package names for installed packages and then excludes the current loaded/running kernel (not necessarily the latest kernel!). This fits in the recommendation of testing a newer kernel before removing older, known-to-work kernels.

So, after upgrading kernels and rebooting to test it, you can remove all other kernels with:

Fantastic answer. One further piece of info would help: How can you tell (for sure) which of the kernels you're using? Presumably you can't just assume it's the last one... Edit: And it looks like the answer is uname -a. Probably best to ensure you don't have a pending restart-to-install-new-kernel pending (as I did; but I noted the mismatch between uname -a and the "latest" kernel).
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T.J. CrowderJun 14 '12 at 8:39

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@freddyb Done. Note that the commands prints/removes kernels other than the loaded/booted one.
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LekensteynAug 13 '12 at 13:57

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@Lekensteyn For me, the GUI warning that /boot is running full comes mostly after upgrades. So you say this would also remove the latest one, that hasn't been booted yet? :/
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freddybSep 20 '12 at 9:58

@byf-ferdy The kernel is an essential part of the OS, if the newest version somehow does not fully work with your hardware, then you can still select a different one to boot from. A nice feature would probably be something that removes all automatically installed kernels older than a month (except for the latest two).
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LekensteynNov 1 '14 at 14:23

Your boot partition is full. Since this is a kernel update, these files will be copied to the boot partition so you need to clean in out. Here is a blog post that will show you how to clear the old kernel images with one command. I'll give a basic synopsis of the method. Use this command to print out the current version of your kernel:

uname -r

Then use this command to print out all the kernels you have installed that aren't your newest kernel:

And that one command will take care of everything for you. I will confirm that this does work perfectly but never trust anybody on the internet. :) For more info, the blog post gives a very good explanation of what each part of the command does so read through it so you are satisfied that it does what you want it to do.

You should be able to highlight all the "linux-" packages with the version "2.6.x" where x is between 31 to 38 according to the files in your /boot folder.

Right-click each of those linux packages and choose the option "Mark for Complete Removal". Finally click the apply button. This will remove all the files and any associated files. Your /boot folder should now be a bit-more tidier.

You can stop using a separate /boot partition, then you won't have such limited space there. To do this, unmount the partition, then mount it somewhere else and copy all of the files there to the /boot directory in your root partition, then remove the entry from /etc/fstab and reinstall grub. For example ( you will need to use the correct partition ):

You can then use gparted to delete the old /boot partition, and possibly extend the root partition to use that space. To extend the root partition you will need to boot from the livecd, and the free space needs to be immediately to the right. If the /boot partition is currently to the left of the root partition, then you will need to first move the root partition to the left, then extend it, but this can take a very, very long time, so may not be worth the trouble.

Moving /boot to my main partition isn't a solution for me since everything but /boot is encrypted. You couldn't know that from my question, sorry.
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user6722Dec 24 '11 at 14:15

This is a good solution if the solutions above don't work due to having no space at all on the boot volume, or if you repeatedly have the "out of space" issue. Please read first why it's sometimes required to have a /boot partition though: (help.ubuntu.com/community/DiskSpace)
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svandragtJul 10 '14 at 8:26

@svandragt, there basically is no reason left to need a /boot partition these days. About the only one left is if your bios is broken and can't see the whole disk, and any machine made in the last decade doesn't have such a limitation. Other legacy cases such as using raid or LVM are now handled properly by grub2.
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psusiJul 11 '14 at 1:59

As the name suggests, apt-get AUTOremove is an automated operation where the system takes its "best guess" what you are trying to achieve. It should not be used as a substitute for common sense system administration. The problem with this approach: If you (or an update) removes a package that is part of this dependency list the auto-remove function wants to remove all other packages in the dependency list, leaving you with a system without working desktop (and sometimes even with a completely broken system).
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RinzwindNov 5 '12 at 10:09

if linux-image-generic is installed and new kernels were not installed explicitly, this will remove old kernels. Common sense is to not accept any changes to the system without reading them first. If common sense has been used prior to using this command then there will be no trouble.
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mchidMar 1 at 23:34

linux-headers-* aren't kernels. Kernel packages are the ones named linux-image-*. The ones named linux-headers-* are development packages for compiling kernel modules: they don't live in the /boot directory and are not required for general day to day use of your system.

The files you listed in /boot do include several old kernel images (vmlinuz*) and compiled initrd images (initrd.img*) for those kernels, which is an indication that you still have a lot of old kernel packages installed.

You should be able to list your installed kernels with

aptitude search ~ilinux-image

(Note that this will probably return packages that aren't kernels, too).

There is usually no need for more than two kernels to be installed - the one currently in use and the previous one to that (as a fallback). So you can start removing the older ones, one by one, like this:

sudo apt-get autoremove linux-image-3.2.0-23-generic

Make sure you substitute "3.2.0-23-generic" with the actual kernel version you want to remove! Also, don't remove packages such as linux-image-generic. You have to be really careful not to remove the currently running kernel or you won't be able to boot (Ubuntu may or may not warn you about doing this).

This is not a good way to remove packages. While this answer isn't very specific about why removing them the correct way failed, I'm sure if that information was given (in a separate question of course) we'd be able to try and figure out the cause of the problem.
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thomasrutterAug 29 '14 at 9:55

If you cannot remove more unused files and if you have other partition with/or free space on same device, you can resize of /boot partition by parted/gparted. (It is included on installation media, too.)

Warning: Resizing of partition is dangerous operation, save your important data on other media before doing it!